Quantitative Analysis of the Effects of Protonation on Tunneling Transport in Molecular Junctions Based on a Benzimidazole-Substituted Terphenylene Dithiol

We report the formation and characterization of molecular tunnel junctions based on self-assembled monolayers (SAMs) of a benzimidazole-substituted terphenylene dithiol in protonated and unprotonated states. Molecular junctions were formed using the conducting probe atomic force microscopy (CP-AFM) platform. The measured current-voltage (I-V) characteristics were well-fitted by the analytical off-resonance single-level model (orSLM), which yielded two key electronic structure parameters, the HOMO-to-Fermi level offset, ϵ_h, and the HOMO-metal coupling, Γ. Protonation of the SAM decreased ϵ_h and increased Γ, leading to an overall increase in junction conductance G using either Au or Pt contacts. Changes in ϵ_h were verified by ultraviolet photoelectron spectroscopy (UPS). In general, the results are consistent with prior reports that tunneling currents through SAM junctions are sensitive to chemical changes. They also demonstrate the effectiveness of the off-resonance single-level model for extracting quantitative electronic structure parameters associated with the junction in its protonated and unprotonated states.